How to convert physics animation to a curve/path?
One method of converting a motion into a path is detailed in How to add trailing light effect to finger tips - by adding a particle system to drop particles as a vertex moves through space and then running a script to convert that particle system into a path. I also created an add-on as part of this answer to make the process easier.

The add-on can be downloaded from here : Particles To Path add-on

Once you've installed and enabled the add-on and you have your trail of particles (disable gravity and interactions from your particle system and have it emit a single particle for each frame of the animation) you simply execute the Particles to Path function (press Space (Blender 2.79) or F3 (Blender 2.8) and start typing the name) :



This should then generate the required path which you can use for your Follow Path constraint.